Micronutrients are generally thought to be essential building blocks for plants and animals. Such micronutrients may include boron, chlorine, chromium, cobalt, copper, iodine, iron, manganese, molybdenum, selenium, and zinc. Micronutrient plant fertilization, alone or in combination with nitrogen, phosphorus, and potash fertilization, generally results in better crop yields and more nutritious food.
Prior micronutrient fertilizers include sulfate salts of the micronutrients, such as zinc sulfate, ferrous sulfate, manganese sulfate and cuprous sulfate or chelates such as zinc EDTA, iron EDTA and copper EDTA. These water-soluble compounds, however, can be leached out of the soil and contaminate water bodies. Other water soluble micronutrient fertilizers have been impregnated with an organic polymer membrane. The nutrients may be released into a solution by diffusion. Still further micronutrient fertilizers have been produced having relatively low micronutrients.
Iron and manganese fertilizers are widely used in many parts of the world particularly in soils of high pH, such as the black soils. They are also widely used in horticultural and cash crops to improve yields and quality of produce. The compounds most popularly used for correcting deficiencies of iron and manganese in crops are ferrous sulphate and manganous sulphate (J. J. Mortvedt, P. M. Giordano & W. L. Lindsay, 1972, Micronutrients in Agriculture, Soil Sci. Soc. Am., Madison). Chelated forms of these micronutrients, e.g., iron-EDTA, and manganese—EDTA compounds are also in use as liquid sprays (V. Saucheli, 1967, Chemistry and technology of fertilizers, Reinhold, N.Y.; G. H. Collins, 1955, Commercial fertilizers, Mc-Graw Hill, N.Y.).
There are, however, several drawbacks in the use of such soluble compounds as fertilizers, e.g., leaching losses, chemical transformation losses, ground water contamination, etc. This results in excess dosages which are often several times the actual crop uptake, leading to poor fertilize-use efficiency. Moreover, such wastages also affect the economics, thereby discouraging their widespread usage.
In an attempt to overcome these shortcomings, slow-release fertilizers incorporating micronutrients have been prepared. One such type is the fertilizer based on phosphate glasses known as frits. Frits are prepared by fusing sodium, potassium or ammonium dihydrogen phosphates together with micronutrient salts at temperatures between 800° and 1400° C. and then rapidly quenching the melt to produce a glass (G. J. Roberts 1973, Am. Ceram. Soc. Bull. Vol 52, p 383; ibid, idem, Vol 54, p 1069; Austrian Patent No 326160 of 1975; U.S. Pat. No. 3,574,591 of 1971; U.S. Pat. No. 2,713,536 of 1974).
The major disadvantage of the phosphate glass frits is that the availability of the nutrients is by slow hydrolysis of the glass and is highly dependent on the soil (pH, moisture content, temperature, etc.,) and on the crop (rate of growth, physiological factors, variety, etc). Consequently, where nutrient release by hydrolysis does not match plant uptake, the fertilizer is not effective. Moreover, the high temperatures involved in the synthesis of frits together with the corrosive conditions make these materials fairly expensive and unsuitable for general use.
Another type of phosphate based water insoluble fertilizer is the metaphosphate. Metaphosphates of calcium and potassium together with micronutrients have been proposed (S I Volfkovich, 1972, J Appl. Chem. (USSR) Vol 45, p 2479). A Russian patent (SU 1270148 of 1986) describes the production of mixed metaphosphates based fertilizers produced at 500-880° C. Drawbacks in the use of metaphosphates as fertilizers are similar to those with the frits. Metaphosphates may be more insoluble and hydrolyse even slower, producing compounds with very poor nutrient availability. Metaphosphates of the heavy metals are extremely insoluble and of little use as fertilizers.
An additional type of slow-release fertilizer has been produced in which the micronutrient ions are in a chemical form wherein they are insoluble but also plant available. These belong to the bio-release types of slow-release fertilizers. The processes for producing such phosphate based zinc and copper fertilizers are described in two Indian patents (Nos 172800 of 1990 and 177205 of 1991). The chemistry of zinc and copper phosphate polymerisation and the chemical nature of these fertilizers have also been described (S K Ray, C Varadachari & K Ghosh, 1993, Ind. Eng. Chem. Res. Vol. 32, p. 1218; S K Ray, C Varadachari & K Ghosh, 1997, J. Agric. Food Chem., vol. 45, p. 1447). A patent (C. Varadachari, Indian Patent Application No. 10/CAL/99) describes the processes for production of slow-release fertilizers and describes methods for assessing limits of polymerisation.